Summary: 2521
SPECIAL FEATURE
Why Omnivory?1
Omnivory, broadly defined as feeding on more than one trophic level, is an ecological habit
that ought to be widespread. At the most basic level, the omnivorous animal, as a generalist,
should have higher performance in variable environments than some more specialized feeder. Yet
the problem of which, why, and how organisms are omnivorous was somehow supplanted by the
apparent observation that omnivory is rare in nature and destabilizing in food webs. From an
historical perspective, Joel E. Cohen and Stuart L. Pimm independently began to advocate the
``rare and destabilizing'' position in the late 1970s. Unfortunately, naturalists who knew better
failed to displace this notion.
Not until the late Gary Polis published his now classic study (G. A. Polis 1991. Complex
trophic interactions in deserts: an empirical critique of food web theory. American Naturalist
138:123­155) was there a resurgence of interest in understanding the biology and ecological
importance of omnivores. The ubiquity of omnivory is now widely recognized, and theory has
demonstrated conditions under which omnivores can stabilize food webs. As theoreticians con-
tinue to work out the influence of omnivory on the stability and lengths of food chains, empiricists
have gotten busy in identifying and studying the causes and consequences of omnivory in nature.
The question about why omnivores have such diversified feeding strategies remains a critical
question that is ripe for diverse hypotheses. Ironically, although arthropods are probably not